A sensor network is being widely used in various real-time application fields, such as forest fire detection, intrusion detection, real-time target trace, emergency relief, and real-time traffic information collection systems. Real-time sensor network applications require the real-time operation on event occurrence, and thus real-time communication characteristic is required as well as low power characteristic which has been required by a related art sensor network application. However, most researches have been concentrated on low-power communication techniques for improving the energy efficiency of the sensor network.
One of the related art low-power communication techniques are using a method that repeats periodic wakeup and sleep, which minimizes undesired operations to decrease power consumption. Generally, energy consumed by communication modules is greater than that consumed through operations. In a sensor network, idle listening of radio frequency (RF) modules has been known to be the biggest energy waste factor. Thus, the energy consumption of a sensor network can be effectively reduced by maintaining a low duty cycle through periodic wakeup and sleep.
According to the periodic wakeup and sleep scheme, however, when a packet is transmitted, it has to wait until the reception node wakes up for each hop. It is called sleep delay, which delays packet transmission due to the sleep state of the reception node. Sleep delay is the main factor of the total transmission delay of packets.
To this end, it has been known that transmission performance is inversely proportional to the energy efficiency of the sensor network. In other words, if a duty cycle increases for enhancing transmission performance, the energy efficiency decreases due to the increased idle listening. If the duty cycle decreases for enhancing the energy efficiency, the transmission performance decreases due to the extended sleep delay.
In such an application as forest fire alarm in which the occurrence of an event should be reported as quickly as possible, transmission performance is a critical issue. The sensor nodes should also last for a long time for preventing the frequent replacement of sensor nodes. A related art sensor network Media Access Control (MAC) protocol cannot satisfy both transmission performance and the energy efficiency that are known to be mutually exclusive.
As another related art, a sequential wakeup approach has been proposed, in which each node sequentially wakes up according to its depth in a routing tree for providing real-time communication. The sequential wakeup approach prevents transmission delay due to sleep delay, thereby improving total transmission performance. The sequential wakeup approach removes channel acquisition contention between nodes having different depths, and thus allows channel contention in only a contention window, removing overhead for transmitting/receiving Request To Send/Clear To Send (RTS/CTS) packet. In this case, however, packet collision may not be avoided when there are multiple traffics. When a packet collision occurs, both transmission delay and energy consumption increase considerably.